Why are they "cheap"?

Well for starters they are Black Powder Express rifles, not nitro rifles.

I don't think they are cheap BTW at $13,000 each for a black powder rifle at all, actually expensive.

In my mind (out of date?) a single .450 BPE DR should be purchasable for about $5,000. These are .500's but don't see a huge increase in price for .500's over .450's in BPE.

But I'm not buying in the USA either, and haven't seen BPE's priced recently.